While on a raid in Afghanistan,  Staff Sergeant Martin Roe takes a picture of a Army First Lieutenant Michael Waters killing what appears to be an unarmed Afghan villager. 

During Waters' subsequent court martial, Roe (who is the key witness against him) suddenly disappears, and the NCIS team is asked to investigate.

During a search of Roe's apartment, Bishop finds a heroin kit, which Abby later discovers contained traces of "China White": a cocktail of heroin and fentanyl.  Additionally, Tony finds a threatening note in one of Roe's dresser drawers.

Later, they find Roe in an abandoned building, dead of an overdoes of China Blue.

It turns out that Roe had been attempting to help a homeless vet, known on the streets as "Blue".  He had taken him to a clinic, but Blue had suffered a heart attack and died.

Roe was later killed by the clinic director - Lorin Davis - because Roe had realized that the clinic had removed Blue's kidneys in order to sell them on the black market.  Bishop and Gibbs arrest Davis.

Meanwhile, Abby searched for Blue's dog, named Dave, and found him with a homeless girl.  After investigating the girl, Abby realized she had run away from home when she was sixteen, and her family had been searching desperately for her.

Abby gave the girl a prepaid telephone card, and showed her a video of her mother pleading for her return.  At first, the girl rejected the idea, but then later on phoned her parents and was reunited with them. The girl asked Abby to take care of Dave "for a while."